1. Field of the Invention
This invention relates to a method and apparatus for controlling the supply of flocculants to a slurry mixture to coagulate various solids in the slurry.
2. Summary Of The Prior Art
In the processing of coal, a feed stream comprising a slurry of coal fines and clay is passed to a thickener or clarifier to which anionic flocculant is added to coagulate the coal fines and cationic flocculant is added to coagulate the clay so that the resulting solids can be separated from the water so that the water can be disposed of or reused in the coal processing.
This type of apparatus for separating solids from liquids may take the form of a large container having a rotary driven rake for moving the agglomerated sludge material from the bottom to an outlet and a weir for removing the clarified water from the top. In many material processing systems, such as coal cleaning, it is desirable to automatically control the addition of the flocculants to the system in response to the concentration and character of the solids in the system.
In prior art devices, a determination of an increase in solids concentration in the slurry will result in an increase in the addition of both flocculants to the slurry which is undesirable since it may be that only one solid, either coal or clay has increased in concentration in the slurry. Thus, it is desirable to know which of the solids of the slurry has increased or decreased in concentration so that the proper flocculant supplied to the slurry can be controlled.
In this area, attempts have been made to determine the character of solids in a coal slurry. A Cendrex continuous ash monitor, developed by the Dutch State Mines, employs a beam of X rays from a cobalt X-ray tube. The beam is split into two equal parts by a rotary chopper, one part striking the coal sample and the other striking a reference of Plexiglas. Because of the difference in absorption coefficients of the coal and the Plexiglas, the intensities of the reflected beams generally are different. This difference is translated by means of a photocell into clay content values for the coal sample.
Another such device was developed by the National Coal Board and the Atomic Energy Research Establishment in Great Britain. This device utilized a radiation source to monitor clay content of a sample slurry. The radiation emitted bombards the surface of the coal, and is either absorbed or backscattered, depending on the elements present in the sample. Elements of low atomic number (the combustible elements) backscatter well, whereas elements of a higher atomic number (clay) absorb the radiation. By measuring the backscattered radiation, a determination of the clay content of the coal sample can be made.
A simple inexpensive system for determining the coal and clay content of a slurry to control the addition of flocculant to a processing cell has not, however, been previously developed.